1. Field of the Invention
This invention relates to a vehicle comprising a binder and at least two solvents for the binder, a printing paste comprising the vehicle and a finely divided, particulate, inorganic coating material, and to the use of the vehicle and printing paste in multiple pass printing.
2. Description of the Prior Art
Particulate, inorganic coating materials are commonly used to seal glass of ceramic parts, such as lamina in a microelectronic package. Sealing is accomplished by applying the particulate material to a substrate, and firing the substrate at an elevated temperature to fuse, vitrify or crystallize the particulate material.
The particulate inorganic coating materials are conventionally applied to substrates by a wet process, that is, dispersed in a binder-solvent system. Such a process typically requires mixing the binder, the solvent and the particulate material to form a paste or slurry which is applied to the surface to be coated. The viscosity of the paste varies with temperature and with the particle size distribution of the inorganic material. Quality control is difficult and the solvent-binder technique is costly in time and material.
One effort to overcome the difficulties inherent in applying a coating or sealant to a substrate via a binder-solvent system is disclosed in U.S. Pat. No. 3,661,615 to Gray and Grier, the entire disclosure of which is incorporated herein by reference.
In U.S. Pat. No. 3,661,615, it is taught that some of the disadvantages inherent in prior art techniques can be avoided by preparing a polymeric based composition which can be used for coating or forming a film on a suitable substrate with incorporation of a solvent and without a change in viscosity and/or thixotropic character. The patent discloses a polymeric based composition containing finely divided, particulate, inorganic material dispersed in a low molecular weight, low vapor pressure, liquid polymer having a relatively constant viscosity and/or thixotropic character during heating of the composition, and which decomposes or pyrolyzes completely to gaseous products at a relatively low temperature without forming and/or leaving a noticeable carbonaceous or other like residue capable of direct and/or indirect deleterious effects. In a specific instance, the polymer is a polystyrene and/or a poly (alpha substituted) styrene having a molecular weight of less than about 1,000.
Unfortunately, the compositions disclosed in U.S. Pat. No. 3,661,615 have not proved entirely satisfactory, especially in the microelectronic package art where pastes are applied to substrates in multiple layers. In the microelectronic package art, the sealant cannot be applied in a single coat because of the difficulty in handling a single, relatively thick film. Rather, the particulate material is applied in several coats with a drying step interposed between each coating step. The solvent is removed from the paste during the drying step. In this manner, a coating of desired thickness is built up from several thin layers. This procedure is commonly referred to as multiple pass printing.
When the sealant is applied in several thin layers, it is essential that the layers adhere to one another. In order to obtain proper adhesion and reproducible products, each layer must be of proper wetness after being heated to remove solvent.
In many instances in which multiple pass printing is employed, a line, image or pattern is being printed, and resolution of the line, image or pattern is an important objective. If a layer is too wet after the heating step, the layer will be too tacky. Furthermore, a layer screened on top of a wet layer tends to slump and flow, thereby adversely affecting resolution of a line, image or pattern. Additionally, if the layer is too wet, pressure from the printing apparatus when a subsequent layer is applied causes the wet layer to compress and lose its shape. There can result a product having an improper thickness.
On the other hand, if each layer is too dry after evaporation of solvent, a layer subsequently screened on the dry layer will pull the dry layer away from its substrate when the printing screen is retracted. This is due to unsatisfactory adhesion between the dry layer and its substrate. Thus, proper wetness is required to obtain reproducible printings having good line resolution and proper thickness.
In the case of the use of the composition disclosed in U.S. Pat. No. 3,661,615, it is difficult to control the wetness of individual layers of sealant after the heating step employed in multiple pass printing.
U.S. Pat. No. 3,625,733 to Mansur suggests the incorporation of a cyclic isoprenoid solvent in the polymer based composition of Gray and Grier. The isoprenoid solvent enhances the wetness of individual layers of sealant in multiple pass coating. Nevertheless, the compositions suggested in U.S. Pat. No. 3,625,733 have not proved entirely satisfactory, especially in the microelectronic package art because of the drying and firing cycles employed in this art. For example, a paste containing the particulate material which is to function as the sealant is applied in a relatively thin layer to a ceramic substrate. The coated substrate is then subjected to a drying step at about 120.degree.-200.degree. F. for about 5-10 minutes to remove solvents in the paste. A second coat of the paste is then applied, and the drying step repeated. This sequence of steps involving application of paste followed by drying can be repeated until the desired thickness of sealant is obtained.
It has been discovered that some of the cyclic isoprenoid solvents are too temperature sensitive during the drying step (i.e., 120.degree.-200.degree.F.), making it difficult to obtain the desired wetness between layers of sealant. The difficulty is apparently due to the vapor pressure of the isoprenoid solvents frequently employed in printing pastes for microelectronic packages. Because of the vapor pressure of these isoprenoid solvents, their rates of evaporation are difficult to control. For example, at the low end of the range of 120.degree.-200.degree. F., the evaporation rate is frequently too low, resulting in the removal of too little solvent during the 5-10 minute heating period. At the upper end of the temperature range, on the other hand, the evaporation rate is sometimes too great. In the first instance, the sealant layer is too wet for subsequent coating, while in the latter event, the layer is too dry. The temperature of the drying apparatus can vary over the range of about 120.degree.-200.degree. F. because of such factors as the number of products per unit time fed to the apparatus, the temperature of the products entering the apparatus, room temperature, whether the products are on trays which will absorb heat, etc.
There are several other problems in the printing paste art which must be considered. Not only must the printing paste for multiple pass printing exhibit relatively constant solvent evaporation characteristics during the drying step, but the paste should have a substantially uniform wetness over its entire surface after the drying step. Further, the paste should flow as little as possible in the absence of pressures above atmospheric pressure in order to avoid contact with undesired areas and to maintain resolution of a line, image or pattern. Additionally, the paste should have as few voids as possible in order that it can be fired to form a strong, hermetic seal.
Thus, it is apparent that there exists a need in the art for an improved printing paste and printing paste vehicle. The printing paste should be suitable for applying a particulate material in several layers on a substrate. It should also be suitable for use where a drying step is interposed between coats. It is desirable that the rate of evaporation of solvents be susceptible to control during such a drying step in order to obtain suitable wetness between layers. It is especially desirable that the rates of evaporation of solvents be substantially constant during a drying step of about 120.degree.-200.degree. F. for about 5-10 minutes. Above all, the paste should have the viscosity, thixotropy and flow characteristics required for the desired use.